Certain commercially available DAB chromogen solutions (e.g. OPTIVIEW® DAB solution, Ventana Medical Systems, Inc. Tucson Ariz.) are stabilized against oxidation. Sodium metabisulfite, for example, has been used as a stabilizing solution at concentrations that do not significantly inhibit DAB tissue staining. Without being limited to a particular theory of operation, this antioxidant is believed (based on elemental analysis) to form an insoluble DAB hydrogen sulfate salt. This DAB hydrogen sulfate salt has low aqueous solubility and readily precipitates from solution. The proposed process is illustrated below.

Tetramethyl benzidine chromogens (TMB) are known to associate with dextran sulfate. Dextran sulfate can maintain TMB oxidized products soluble under processes normally used to precipitate TMB chromogen products in solution and also facilitate the TMB deposition in western blotting. [Proc. Natl. Acad. Sci. USA Vol. 83, pp. 9085-9089: “Use of nonisotopic M13 probes for genetic analysis—Application to HLV class II loci”, U.S. Pat. No. 4,789,630—“Ionic compounds containing the cationic meriquinone of a benzidine” and US5013646 or WO199100667—“TMB formulation for soluble and precipitable HRP-ELISA”]. There still exists a need in the art, however, to address the problem of DAB-sulfate precipitation prior to DAB oxidation in tissue staining methods.